Sealing plastic film around objects provides an inexpensive packaging alternative to individually made and designed packaging such as paper or plastic cartons. The film in effect becomes an individual packaging container in the form of a scaled bag. Various types of films are used in this packaging method. The film may be stretch film, shrink film, or hard film, which neither stretches readily nor is adapted to heat shrinking significantly. Many different items are packaged in these plastic bags such as meat and poultry parts, which are `trayed` then bagged in the plastic film. The "bags" are commonly formed from a continuous roll of film sheet around the trayed items.
With reference to FIG. 1A, there is shown schematically a typical packaging machine as indicated by reference number 10. The conventional product-packaging machine 10 as shown utilizes a continuous roll 1 of heat sealable film 2. A conveyor moves the product trays 3 overwrapped with film 2 in a longitudinal direction. The overwrapped tray is fed over various symmetrically arranged rollers 4 which fold the overwrapping film to give it a tubular form, with the two side edges of the film mating along a vertical plane longitudinal with the running direction of the film 2. As the overwrapped tray 2 is passed over the sealing apparatus 20, the heat sealing wheels of the apparatus 20 (at a pre-selected temperature) welds the mating edges of film longitudinally below the overwrapped tray 2 forming a tubed tray 5, with a bottom seal on each successive packaged tray. The sealed tube 5 is then sealed (or cut and sealed) in the direction transverse to the direction of the moving tube and tray by transverse sealer 6, forming individually packaged trays. The direction in which the film, the packaged product, and the heat seal below the packaged tray forming a sealed tube travels is referred to as the `machine` direction or the `longitudinal` direction. The direction of sealing and cutting of the sealed tube forming individually packaged trays is called the `transverse` or `cross` direction.
The above description for transport of the products, synchronization, and folding of the plastic film is known in the art and common in many continuous packaging machines, and is not considered necessary to explain here in greater detail. However, there are major differences in the apparatuses used in forming the longitudinal seals. Published patent application no. WO 93/23234 entitled "Vacuum Advance System for High Speed Contact Sealer for Forming a Flat Film into a Tube," discloses a longitudinal sealing system employing a heated endless band which passes over a heater bar and rollers at each end thereof. The heated band is impinged directly against the seam defined by the overlapping edges of the film, and the heater bar effects the sealing of the seam. Adhesive applicators or rollers can also be used in this sealing system to further effect the sealing of the overlapping film edges.
U.S. Pat. No. 4,746,391 similarly discloses the use of an electrically heated heating band to seal the folded film under heat and pressure applied by a heated band. Electrical current is supplied to endlessly revolving heating band via feed rollers and from a current source for resistance heating. As a result of thermal conduction via the steel heating band, the sealing heat is transmitted to the films to be sealed or welded to one another.
Another approach to longitudinally heat-sealing plastic film is passing the packaging film through a heated nip formed by two seal wheels as shown in FIG. 1. The heat seal wheels of the prior art are illustrated in FIGS. 2A and 2B. In the prior art, heat is provided to each rotating seal wheel 11 by a donutshaped electrical resistance heater 13. The heater 13 is a metal block cast around a coiled electrical heating element with all surfaces of the heater 13 being essentially at the same heating temperature. Seal wheel 11 extends downward defining a circular skirt 12 around heater block 13. Skirt 12 is necessary to keep excess trims from sticking to and melting on heater block surface. Trims are lateral cut-off edges of overlapped film. Heat is transferred from heater block 13 to seal wheel 11 through a metal disk 14, which acts as a thrust bearing. Compression springs (not shown) arranged equally spaced around a circle concentric to the heater axis, force the heater block 13, metal bearing disk 14, and seal wheel 11 in contact with one another to facilitate heat transfer.
As packages overwrapped with plastic film move on a conveyor belt over the heated seal wheels with the edges of the film extend down in the elongate direction overlapping one another, the two heated wheels 11 apply pressure and heat on the overlapped edges of the film forming a longitudinal seal underneath the packages. As the seal is being formed, the rotating movement of the seal wheels nips and severs the trim from the longitudinal seal line formed underneath the overwrapped packages.
There are various problems frequently encountered with the heat seal wheel assembly of the prior art. One is that it takes a long time to start-up the conveyor line. As the rotating surfaces of the seal wheels 11 are heated, some of that heat is also transferred to the wheel skirt 12 and the surrounding environment. Thus, more energy and time is required to heat the sealing edges of the seal wheels 11 to a predetermined temperature as the result of the convection and radiation heat loss from the heating block 13 to the surroundings. The other problems are maintenance in nature. The bearing disk 14, being sandwiched between the rotating seal wheel 11 and heating block 13, has to be kept greased and replaced on a frequent basis due to wear thus causing frequent shut-downs. Another frequent cause for shut-downs is due to trim wraps. As the seal wheel 11 is heated, the outer surface of seal wheel skirt 12 is heated as well due to its close proximity to heater block 13. It has been found that when trim tension is lost, the trim or excess lateral cut-off edges of overlapped film will curl, melt and stick to the rotating wheel skirt 12 and shut down the machine. Machine operator has to clear the trayed products off the line, remove the trip wraps, and restart the machine.
Accordingly, it is an object of this invention to provide a novel construction for a seal wheel heat transfer system that is simple in design and operation, and eliminates costly shut-down problems experienced in the prior art.